Circuit-switched connections, whether in analog or digital transmission, have traditionally been used for voice connections. A circuit-switched connection, however, is inefficient as regards transmission efficiency. A communications connection, once established, remains reserved even if there is nothing to transmit.
Data can also be transmitted using packet-switched technology. Such a type of technology has long been used in communication between computers. The Internet is one example of how data can be transmitted from one place to another using packet-based communication. In a packet-switched network the data to be transmitted are arranged in blocks/packets of certain length. Before sending, at least the destination address is additionally inserted in each packet. Thus, in addition to the user data proper, whether it be voice or something else, also other data are being transmitted at the same time. Such supplementary data are inserted in the header fields of the packets transmitted.
With some applications the size of the header fields becomes fairly large in comparison with the user data proper. This problem is emphasized by the fact that the packets comprise transmission protocols layered on top of each other, which protocols all have header field needs and forms of their own. Examples of such interrelated transmission protocols with their header fields include the Internet Protocol (IP), User Datagram Protocol (UDP) and the Real Time Protocol (RTP) which can be used e.g. to establish a real-time packet-switched voice connection between parties. In this example, as well, the layered protocol structure naturally increases the header field data.
Attempts have been made to alleviate this problem through frame compression. Frame compression aims to remove the header-related data that are identical in successive frames, or that are such that the information associated can be easily derived from the packets received previously. One such compression method according to the prior art is called Robust Header Compression (ROHC). Even with ROHC applied, the frame header data result in a considerable increase in data to be transmitted. For example, in a VoIP (Voice over IP) call, a ROHC-compressed voice packet could include 15 bytes of voice samples and 4 bytes of data related to header fields. In other words, the proportion of header data in all data transmitted is fairly large in spite of the compression. When such a call is directed onto a bandwidth-limited transmission path, problems ensue. For instance, in a GPRS (General Packet Radio Service) network the transmission band on the radio path may prove to be narrow compared to the transmission capacity required.
With compression methods, uncompressed frames are usually used in conjunction with the first packets transmitted. For example, in Degermark compression, which is used for IP packets, the packets numbered 1, 3, 6, 11, 20, . . . have uncompressed header fields. Once continuous packet transmission has been started after the beginning of a speech spurt, the transmission of uncompressed header fields can be decreased. Then, for example, every 64th header field will be uncompressed.
One possible way of establishing a connection over a packet-switched GPRS network is to use PoC (Push to talk over Cellular). Several people may participate in a PoC session simultaneously. People take turns speaking, and the direction of transmission varies. There is thus a great need to transmit uncompressed header fields during a PoC connection. A PoC connection may use one coding method CS-1 (Coding Scheme) allowed in the GPRS. This coding scheme allows for a large cell size at the cost of transmission speed. As the nature of PoC is such that a great number of packets are transmitted in a session with uncompressed header data, the coding scheme selected may result in that the radio channel used may prove small always during the first packets sent in a particular direction. Therefore, the bit rate used on a PoC radio channel varies greatly and in some cases it may exceed the maximum available transmission capacity. Later on, the bit rate required by the PoC connection is decreased because frame compression can be effectively utilized. As regards capacity-limited radio path, this situation is not optimal. In an optimum situation, the radio channel is loaded as evenly as possible throughout the whole connection time available.
Digital circuit-switched connections utilize so-called frame stealing. This means that data blocks reserved for voice samples are stolen here and there for urgent data transmission needs. At a receiver end, no speech signal is thus received in these time slots. However, the listener receiving the voice sample will not perceive that a voice sample has been removed from the signal received. Using frame stealing, for example in the GSM (Global System for Mobile communications) networks, it is possible in urgent cases to allocate the FACCH (Fast Associated Control Channel) onto the frames that have been intended for voice samples frames.